Internal combustion engines use fuel pumps for delivery of fuel to combustion chambers of the engine. Fuel pumps include mechanical and electrical types. As shown in FIG. 1A, a mechanical fuel pump 10 includes a pump actuator 12 disposed in a housing 14. The fuel pump 10 includes a cam assembly 16 rotatably positioned in the housing 14 in proximity to the pump actuator 12. The cam assembly 16 includes a cam 22 mounted on a cam shaft 18. The cam 22 defines a cam surface 22B.
As shown in FIG. 1B, the pump actuator 12 defines a substantially cylindrical body 12B that has an interior surface 12C. A shoe 30 having a circumferential face 30C is press fit into the interior surface 12C. The actuator 12 slides (e.g., up and down or in and out) of a bore 14B defined by the housing 14. The cam 22 has an axial width W2. The shoe 30 has an arcuate (e.g., circular contour) seating surface 34 extending diametrically across the shoe 30. The seating surface 34 is open a width W1 at an axial face 32. The arcuate seating surface 34 extends axially into the shoe 30 a depth D1, from the axial face 32 of the shoe 30. A roller 20 is rotatably disposed on the seating surface 34 so that a portion of the roller 20 extends outwardly from the axial face 32 of the shoe 30. The roller 20 has a diameter D2 that is greater than the depth D1. In one embodiment, the width W1 (in this particular case shown) is less than the roller diameter D1, such that the roller 20 has to be inserted axially and cannot fall out radially. This eases assembly/disassembly and avoids the roller 20 falling out should the roller leave the cam during operation. The roller 20 defines a cylindrical exterior surface 24 that extends an overall length LX from a first axial end 20X to a second axial end 20Y of the roller 20.
The exterior surface 24 of the roller 20 is rotatable relative to the seating surface 34 of the shoe 30. In particular, the exterior surface 24 rotates hydro-dynamically on a hydraulic wedge of lubricant 40 in the seating surface 34 of the shoe 30, as shown in FIG. 3. The wedge 40 lifts the roller 20 away from the shoe 30 so that during operation, there is negligible contact between the exterior surface 24 and the seating surface 34. However, to achieve adequate hydro-dynamic wedging and lift, an adequate length of roller is required.
As shown in FIG. 4, the exterior surface 24 of the roller 20 rotates in the seating surface 34 of the shoe 30 about an axis A1 in a direction indicated by the arrow J1, while the roller 20 translates in the shoe 30 in a direction indicated by the arrow K. The exterior surface 24 of the roller 20 rolls on the cam surface 22B as the cam 22 rotates around an axis A2 in the direction indicated by an arrow J2, which is opposite to the direction J1. As a result of the rolling of the exterior surface 24 of the roller 20 on the cam surface 22B, edge portions 50A and 50B of the exterior surface 24, proximate the axial end 20X and 20Y, respectively are subject to high contact pressures and subsequent subsurface stresses in the material and tend to fail prematurely. The graph 500 of FIG. 5 indicates peak contact pressures at planes 501 and 502 of the roller 20, proximate the first axial end 20X to a second axial end 20Y of the roller 20. The peak contact pressures at planes 501 and 502 is about 140 percent of the contact pressure at an intermediate plane 503 of the roller 20. Attempts to reduce the high stress in the edge portions 50A and 50B have been undermined by the contrary need to maximize the overall length LX of the roller 20 to achieve adequate hydro-dynamic wedging and lift. In addition, efforts to maximize the length of the roller 20 to achieve adequate hydro-dynamic wedging and lift has worsened the stress in the end portions 50A and 50B.
Based on the foregoing, it is the general object of this invention to provide a roller and roller profile that optimize hydro-dynamic wedging and lift, and reduces stress on the end portions 50A and 50B of the roller/cam contact.